We are in the process of completing a 1+ year project that started off purely MVC 3 and Telerik Extensions for MVC (Kendo didn't exist yet then) and we have since migrated out whole dashboard to Kendo UI.
Your requirements are very similar to what we are trying to accomplish. We haven't gotten to the mobile part yet but it's well on our roadmap. Basically what we use today is all our transactional data is managed by Entity Framework and Web API. Everything on our site from registration form on the www. to every single drop down on our dashboard. uses JSON data from Web API. For the dashboard we use the Kendo MVC Extensions, simply because it's easier and quicker to write things and also migrating from the Telerik to Kendo controls was a no-brainer for the most part.
JsonResult MyActionJson([DataSourceRequest]DataSourceRequest request)
var data =
Basically, you will find that you'll end up struggling to implement a lot of your own stuff if you chose to not use the MVC extensions. It's not just Html helpers - their IQueryable extensions are really good and make things very simple. Even in our case with custom sorting and filtering, we were able to easily write helpers to map and remap our sorts and filters to their SortDescriptors and FilterDescriptors.
Things to keep in mind with Kendo is that the Kendo DataSource (which is the core for all data binding of controls) does not interpret the types of related objects when using AJAX binding with JSON so you will need to use projection to flatten your business objects and complex types to simple ViewModels before sending them to your client and then you need to remap your ViewModels back to your business objects. We use AutoMapper
for our projections of properties, filters and sorts. I can send you source code samples (and I'm trying to release some of our stuff like this too in their code library if I get approval) if you're interested.
I don't see a single reason for you to not use Web API for everything from desktop to mobile though. It is an extremely flexible solution, you can have a single source for all your data that way and not have to re-invent the wheel each time for desktop or mobile. Alternatively, if your data needs to vary so much between mobile and desktop devices, there are Web API solutions for that that route to different Mobile controller if the request is coming from a mobile device.
As to what Pat Tormey is talking about having to "jury rig" the autocomplete box to get consistent styling, I am not sure unless he's not aware of the Kendo CSS classes such as k-input, k-button, etc. Sounds like a classic example that you need to learn and know your tools. They have great documentation site and examples and are adding new stuff all the time. Six months ago you had to literally dig through .js and .css files to figure stuff out. Now they have even documented their ASP.NET IQueryable extensions and every little class and interface they use.